A

According to Hook's Law, the force exerted by a spring is directly proportional to the displacement from its equilibrium position. The law is mathematically expressed as:
$$ F = kx $$
where:
- F is the force applied on the spring,
- k is the spring constant (a measure of the stiffness of the spring),
- x is the displacement from the original length.
In this case, if the spring is extended to a length \( l \), we can assume \( x = l \). Therefore, substituting \( x \) into the formula gives us:
$$ F = kl $$
This corresponds to option (A) in the provided choices.
Therefore, the correct answer is A.